When producing pulp in accordance with the sulphate pulp method, the wood is digested in the form of chippings in an alkaline solution chiefly consisting of sodium hydroxide and sodium hydrosulphide. The relative proportions of hydroxide ions [OH.sup.- ] and sulphide ions [HS.sup.- ] vary from plant to plant but normally within the interval 25-40% sulphide [2HS.sup.- ], calculated on the basis of the sum of the contents of sulphide and hydroxide ions.
An increased content of sulphide ions in relation to hydroxide ions in the cooking liquid increases the pulp yield and normally improves the important properties of the final product.
At the beginning of the cooking process, the so-called impregnation phase, an increased proportion of sulphide ions provides the possibility of achieving a more efficacious dissolution of lignin during the cooking process itself. A lower content of hydroxide ions in this part of the cooking process decreases the breakdown of cellulose, thereby increasing the yield and improving the quality of the final product.
Recently, the cooking process has been modified in order to increase the yield of the pulp and to improve its properties. This has been done by dividing up the alkali charge which is required by adding a part of the cooking liquid (the white liquor) in the conventional manner together with the wood and then adding the remaining quantity later in the cooking process. However, the ratio between the sulphide and hydroxide ions remains the same in the white liquor which is added. In certain cases, an increased recirculation of spent liquor (black liquor) to the beginning of the cooking process has been effected with a view to obtaining a higher content of suliphide ions in this stage of the cooking.
The present invention provides the possibility of apportioning sulphide and hydroxide ions within wide limits by dividing up the available alkali after the liquor combustion into two or more constituent streams.